What’s Inside a Substation Transformer? A Look at the Major Parts

Substation transformers step power down from high-voltage lines to the lower voltages used on distribution circuits. The one featured in this lecture takes 230 kV down to 23 kV, and a transformer that size typically supplies between one and six distribution circuits.

They’re also the biggest, most complex piece of equipment in the substation. As Tim Taylor, instructor of HeatSpring’s “Utility Distribution Systems” course, explains, real substation transformers are much more complex than the simple two-coil diagrams used to teach how transformers work. They include elaborate cooling and insulation systems built around the core.

In this clip, Tim walks through the major components: the core and windings, the oil-filled tank, the bushings that keep the tank safe to touch, the radiators and fans that handle cooling, and the oil conservator that gives the insulating oil room to expand.

“Utility Distribution Systems” is built for solar and energy professionals who work alongside utilities and need a solid understanding of how the grid is engineered and operated.  Tim brings more than 35 years of utility experience to six modules covering DER interconnection, voltage control, overcurrent protection, and more. Enroll your team in Utility Distribution Systems!

Transcript below

So with substation transformers, I’ve shown a picture here, and this is a relatively large one. I recall this one being twenty MVA or so, and it was actually stepping voltage down from a 230 kV line down to 23 kV. It typically supplies between one to six distribution circuits. A big substation transformer of this size is the biggest, most complex piece of equipment in the substation, and on the next slide, we will get to what some of those components actually are.

So in this diagram, in the middle of the picture, you’ve got the core and the windings that I just discussed. In this three-phase transformer, you should see three cores and windings, but you can’t really see it in this diagram. In terms of the cores, there’s two different types shown over on the right: the core type and the shell type transformer. With the core type, you’ve got two coils and one core, and with the shell type, you’ve got two cores and one coil. I wouldn’t worry about that too much unless you’re actually specing out power transformers, but it’s good to know that there’s different nomenclature and different types of construction there.

Surrounding the windings and the core, you’ve got this tank. The tank obviously holds in all of the insulating oil that is within the transformer, and of course, it protects the core and coil from outside damage that could occur.

On the top of the transformer here, you’ve got these things called bushings. What the bushings do is enable that high voltage that’s coming into or out of the transformer to be essentially isolated from the tank itself, because the tank is at ground potential, meaning that under normal circumstances, if someone was to go up and touch that tank, they wouldn’t get electrocuted. The bushing is the device that enables that transition from the transformer up to the transmission line or distribution line and not energize the tank.

Some other parts: you see radiators and fans on large power transformers. You don’t see them on distribution transformers, although once you get up into bigger sizes of distribution transformers, you might see them. These provide the cooling, which is required because the current flowing through the coils is producing a lot of heat, a lot of I²R losses. So the radiators and fans can cool the transformer, and it helps the manufacturer keep the transformer size down.

You’ve got an oil conservator, which is kind of like an overflow. Well, it’s not really an overflow device, but it allows for oil expansion to occur as the oil heats up.

Oh, by the way, a key quiz question will be: does anyone know why transformers make a humming sound? That’s because they don’t know the words to the song.